458 
BULLETIN OF THE BUEEAU OF FISHEKIES. 
end of the palp. It will be noted that there is a conspicuous notch or depression 
on the posterior side of each ridge near the bottom and another, not as large and 
well defined, on the anterior side and near the top of the ridge. The notches are, 
of course, cross sections of longitudinal depressions or grooves along the sides of 
the ridges. The epithelial cells in the bottom of one of these depressions are 
scarcely half as high as those covering the remainder of the ridge. The cilia there 
are short. The posterior notches are shown by Allen (1914, fig. 13) but are not 
commented upon. The anterior notches are not shown. No further reference to 
these depressions has been found in the literature on the subject. 
It can further be seen (figs. 15 and 16) that the epithelial cells and cilia are both 
relatively long from the posterior notch upward to a point just anterior to the crest 
of the ridge, and from this point downward to the anterior notch they are shorter. 
Immediately below the anterior notch is a group of longer cells and cilia, while below 
this the cells and cilia are short. In the bottom of the furrow between the ridges 
the cells and cilia are longer again, and from the bottom upward on the posterior 
side of the ridge to the posterior notch the cells and cilia are as long as at any point 
on the palp. These structural details may be observed in specimens as small as 
7 mm. (figs. 10 and 13 show them to some extent) but are more conspicuous in the 
adult. 
THEORY OF THE FUNCTION OF GROOVES ON RIDGES OF PALPS. 
What is the connection between the anatomical features just described and 
the observations made of the actual process of ingestion, especially those made in 
the mussel 10 to 12 mm. long wherein these structures are present? By the use 
of the strong transmitted electric light, as described, particles were seen to pass 
across the inner face of the outer palp, from ridge to ridge, to the lips and mouth. 
Other particles were seen to pass down the furrows between the ridges to the ventral 
edge of the palp. Both activities were usually occurring simultaneously, either one 
to a greater or less degree according to circumstances. Particles were never seen to 
pass across the palp in a posterior direction, and particles were never seen to pass 
up the grooves toward the dorsal edge of the palp (Siebert, 1913, and Wallengren, 
1905) . There was never the slighest evidence of a reversal in the direction of beat 
of the cilia, even when carmine grains were touching the mantle edge, palp, 
mouth, esophagus, or stomach. The valves often closed, apparently to exclude 
carmine, either at once or after some had been ingested, but no carmine was ever 
seen to move backward across the palp. Carmine was seen to move down the 
grooves, as were all sorts of other particles, often at the same time that other 
carmine grains were moving forward across the palp. 
It would seem that the anatomical and behavioristic evidence would support 
the following theory concerning the action of the cilia. The cilia in the area over 
the posterior side and crest of the ridge between the posterior and anterior notches 
always beat forward, moving particles toward the mouth, the ridges leaning well 
forward. The cells and cilia in most of this area are long. Just above the anterior 
groove, however, the cilia are short, and these would allow some particles in the 
process of transfer to the next ridge to fall down between. If the palp were stretched 
